Magnetic field sensors are used for measuring magnetic fields. These are sensors which convert the magnetic flux or the magnetic field strength into an electrical quantity. Since magnetic field sensors are contactless, reliable and wear-free sensors, which can be produced relatively cheaply, they are frequently used for the detection of other physical quantities: for pressure, for fill level, rotational speed, current, rotational movement, torque, directional changes, and others. Magnetic field sensors include, inter alia, Hall sensors and magneto-resistive sensors. So-called fluxgate sensors, also called saturation core probes or Foerster probes, are high-sensitivity magnetic field sensors. The magneto-elastic technology uses basic mechanical and magnetic properties of the material to measure various parameters. In this case, measurements are made of changes in the properties of magnetic fields which accompany a change in the mechanical properties, for example, the shear stressing under the action of external forces on a magnetic component. The technology is applied by directly magnetizing a mechanical elements instead of attaching additional elements such as, for example, a ring. Highly sensitive fluxgate sensors which are located in the immediate vicinity of the magnetized element identify the change in the magnetic field properties which are proportional to the applied force. These changes are linear and reproducible within the elastic limits of the material. They are precise under normal application conditions.
Examples for this technology overall are found in numerous patent specifications. They can be designated as product-related magnetic fields since they usually act on the magnetic field of the respective product.
An apparatus for measuring forces or moments is already deduced from DE 277 521 A1. According to this, a measuring device is described which accomplishes the contactless measurement of forces or moments at fixed or rotating deformation bodies on the basis of the magneto-elastic effect. The deformation body of the apparatus comprises the deformation body itself, at least two zones of thin amorphous metal layers applied to the surface of the body and sensor coils assigned to these zones. The measuring device measures surface stresses as a consequence of force or moment effects on cantilevers, force measuring bolts, torsion shafts, etc.
In this case, a sensor coil is assigned to each surface zone of the deformation body which either surrounds the deformation body in the region of the coated surface zone or is arranged in the form of an air coil or at the shortest possible distance from the surface zone.
Further deduced from DE 10 2005 002 966 A1 is an apparatus for determining power of operating means which provides the use of additional redundant sensors for measurement of the torque present on a shaft part.
EP 2 793 009 A1 additionally describes an elongate shaft which has magneto-active fields which are magnetized radially in opposite directions. These magnetized fields are assigned magneto-elastic sensors. The shaft including its magneto-elastic fields is subjected to a tensile and/or compressive loading.
Known from U.S. Pat. No. 8,578,794 A1 is a torque sensor which comprises an elongate body and magneto-elastic active fields which are connected directly or indirectly to the body or which form a part of the body. A torque exerted on the body is transferred proportionally to the active field which includes at least one magnetically polarized field. Signals of the magnetic field sensor are processed in such a manner that effects of a magnetic near field are compensated.
Furthermore EP 2 799 827 A1 discloses a magneto-elastic torque sensor comprising a hollow elongate body at least in some regions and a magneto-elastic active field acting on the inner surface of the body. The torque sensor comprises a plurality of magnetic field sensors which are arranged in the interior of the hollow body in some regions.
U.S. Pat. No. 6,553,847 B2 deduces a torque sensor, in particular a contactless magneto-elastic torque sensor for providing a measure for the torque which is applied to a shaft.
A torque sensor comprises a magneto-elastically active region on a shaft exposed to a torque and a magnetic field sensor.
The magnetic field sensor is attached in the vicinity of the magneto-elastic region and is oriented in relation to this in order to detect the magnitude of the magnetic field of the sensor device and provide the output signal as a response to this.
US 2012/0074933 A1 discloses a method and an apparatus for eliminating a field noise in a torque sensor. For this purpose three sets of magnetic field sensors and relevant coils are placed on a shaft. The first set is arranged in a central region of the shaft. The second and the third set of field sensors are each arranged on the right and the left side of the field sensor of the central region. All the coils are connected to one another and jointly emit a sensor signal. According to this prior art however, only the end result can be identified. There is no scope for differentiated identification in relation to the basic parameters such as, for example, torque, stress etc. There is also no scope for determining what is actually happening.
Known from US 2011/0162464 A1 is an apparatus comprising at least two magnetic field sensors wherein in addition to a product-related magnetic field, an external magnetic field can be compensated. In this case, sensor arrangements comprising one set of sensors and comprising a plurality of sets of sensors are disclosed, wherein one set of sensors is localized in the region of a magnetic track or in the region of a plurality of magnetic tracks and the other sets of sensors are localized close by but without any specific assignment of individual coils to individual magnetic tracks.
The document US 2013/0125669 A1 also discloses an apparatus comprising at least two magnetic field sensors and discloses a measurement arrangement comprising a pair of sensors and comprising a set of sensors consisting of three sensors, wherein each sensor is assigned a coil and an appurtenant magnetic track; furthermore, the possibility of a corresponding expansion with other pairs of sensors or other sets of sensors each consisting of three sensors is addressed.
The document U.S. Pat. No. 8,001,849 B2 discloses sensor arrangements comprising a plurality of sensors wherein at least one sensor is localized in the region of a magneto-elastic zone and at least one further sensor is localized close by or further away but without a specific assignment of individual coils to individual magnetic tracks.